Up to one-third of a building's heat is lost through windows. Double and triple glass panes are now frequently specified to combat this heat loss. Many manufacturers are attempting to increase the "R" value (resistance to heat conduction) in their window frames and mullions. Some metal window frames are now fabricated with thermal breaks, or gaps between opposed metal frame components, to reduce heat conduction as well as condensation on the room side of the frame. Wood window frames are also being specially designed to reduce heat loss and condensation. Wood is not as dimensionally stable as metal and it can dry out and warp. This can produce cracks allowing air infiltration and heat loss.